package com.lcm.oj.lanqiao.year2021.moni.c1;

import java.math.BigDecimal;
import java.util.Scanner;

/**
 * @author Coder-lcm
 * @date 2021/4/17
 */
public class T7 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int l = scanner.nextInt();
        int h = scanner.nextInt();

        BigDecimal b = new BigDecimal((float)l * h / 2);
        System.out.println(b.setScale(1, BigDecimal.ROUND_HALF_UP).doubleValue());
    }
}
